The present invention relates to microelectrodes and to a process for shielding the same.
Microelectrodes are widely used in electrophysiology for recording potentials, passing currents or for iontophoresis. These microelectrodes are usually fabricated of glass pipettes having a central opening therethrough, the tip of which is adapted to be placed in a tissue bath or the like for the above-mentioned sensing or probing purposes. However, at audio frequencies, the electrode capacitance to the tissue bath and to adjacent electrodes can cause interference and reduced fidelity. In an attempt to overcome these problems various techniques of or forms of electrical shielding have been proposed.
For example, the provision of a driven shield over a voltage recording electrode has the desirable effect of reducing the effective capacitance of the electrode to thereby permit higher bandwidth recordings. During iontophoresis and recording employing a multiple barrel pipette or electrode, the response time of the recording pipette is reduced due to the pipettes capacitance to the neighboring iontophoretic barrels as well as its capacitance to the tissue bath. In addition, voltage fluctuations generated by the passage of current through the iontophoretic barrels can cause electrical noise in the recording barrel. Electrodes are known wherein a graphite aerosol has been used to shield the recording microelectrode to within 1 mm of the tip thereof. For a typical microelectrode-to-bath capacity of 1 .sub.p F/mm, however, this 1 mm tip exposure produces too much capacitance for more demanding application such as microelectrode voltage clamping or high fidelity impedance measuring.
One of the major drawbacks in the employment of microelectrode voltage clamps is the unavoidable capacitance between the current and the voltage electrodes which combines with the resistance thereof to form an R C circuit to thereby introduce a time lag or constant that significantly slows down the response or recording process. Electronic compensation has been proposed, in an attempt to overcome this problem, by partially cancelling the interelectrode capacities. However, the success of such electrode compensation techniques is subject to the variabilities in the electrodes themselves, foremost among which is the resistance of the current electrode. Further, such compensation techniques introduce excessive noise in that amplifier noise fed through the compensation capacity is added to the membrane potential. It is, therefore, extremely crucial to reduce or minimize the capacity between the current and voltage electrodes.
Heretofore, the common method of shielding microelectrodes is to simply apply silver paint to the same with the aid of a suitably powered microscope. However, the grain size of silver paint is large and the painting process is tedious, especially if it is desired to shield close to the tip of the microelectrode.